docs: def redirect_display(self, on):
          '''
          on:
           * True -> set $DISPLAY to virtual screen
           * False -> set $DISPLAY to original screen
  
          :param on: bool
          '''
          d = self.new_display_var if on else self.old_display_var
          if d is None:
              log.debug('unset DISPLAY')
              del os.environ['DISPLAY']
          else:
              log.debug('DISPLAY=%s', d)
              os.environ['DISPLAY'] = d